JPS6230897B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a new polyester film, and more particularly to a polyethylene terephthalate film or a polyester film containing ethylene terephthalate units as a main constituent, which has improved abrasion resistance and thermal dimensional stability and is particularly suitable for use in magnetic tapes. Polyester film can be strengthened by biaxial stretching and heat treatment, and can also be given excellent properties such as dimensional stability, transparency, chemical resistance, and electrical properties, and can be used for magnetic tape, It is widely used in a variety of fields, including for capacitors and other electrical insulating materials, and for manufacturing gold and silver thread. Among these uses, the demand for base films for magnetic tapes for audio, video, computers, etc. is increasing year by year, and along with the increase in demand, the quality requirements for the base films are becoming increasingly strict. Strict requirements are placed on such base films in terms of not only strength but also various properties such as flatness of the film and foreign matter adhering to the film surface. Among these, the abrasion resistance and dimensional stability of the magnetic tape base film are important factors.
Thermal dimensional stability is a very important property in order to withstand friction with guide rolls during repeated playback use of recorded tapes, and thermal dimensional stability improves yield in the magnetic tape manufacturing process and also helps maintain the performance of magnetic tapes. This is an important property. When magnetic tape is used repeatedly for recording, recording, playback, etc., the tape comes into contact with various guide rolls,
The surface is abraded and so-called white powder is produced, and these white powders cause noise and dropouts and degrade tape performance. In addition, this type of phenomenon also occurs in the process of coating magnetic films, and similar to the above phenomenon, noise and
This has become a major problem as it causes dropout and greatly reduces the yield in the coating process. A known method for improving abrasion resistance is to roughen the film surface and reduce the contact area between the films by adding relatively coarse particles. It has been found to be more effective. However, increasing the size of the additive particles beyond a certain level not only deteriorates the electromagnetic conversion characteristics, which is an important characteristic of magnetic tape, but also increases the amount of particles added due to voids formed around the particles during film stretching. It becomes easy to separate from the polyester body, and the separated particles cause dropouts and noise, which not only causes a loss of performance as a magnetic tape, but also causes the loss of valuable information in computer tapes. Another known method is to lower the heat treatment temperature after stretching the film to reduce crystallinity, thereby improving wear resistance, which is an important characteristic of magnetic tape base films. This will worsen the dimensional stability, and in audio tapes, the skew characteristics will deteriorate.
Errors can occur in computer tapes, causing problems when used as magnetic tapes.Furthermore, if the heat treatment temperature is lowered, the slitting characteristics when slitting the film will not only drop, but the tape The quality of the product also deteriorates, making it less practical. In addition, it is known that raising the heat treatment temperature or applying relaxation during heat treatment is a method for improving thermal dimensional stability, which is another important property. As a result, not only the abrasion resistance decreases, but also the strength.Furthermore, when the relaxation treatment is performed, the dispersion of physical properties in the film width direction increases significantly, reducing the yield in the film manufacturing process, and furthermore, the strength in the relaxation direction decreases. As a result, the residual strain is large when tension is applied, which significantly worsens the skew characteristics when running the magnetic tape, resulting in a significant drop in quality. In this way, wear resistance and dimensional stability are contradictory factors, so it is difficult to satisfy both at the same time, and the reality is that conventionally, one of the more important factors has been the focus. It is. In view of this background, the present inventors have conducted extensive research in order to obtain a high-performance film as a base film for magnetic tape, and have finally completed the present invention. An object of the present invention is to provide a base film suitable for magnetic tapes that simultaneously satisfies abrasion resistance and thermal dimensional stability when used for magnetic tapes without impairing other properties. The gist of this is that the surface hardness (Hh) (Kg/cm ² ) of one side of the film is 530Hh<880, and the surface hardness (Hl) (Kg/cm ² ) of the other side is Hl500. The thickness (tl) of the layer with low surface hardening is tl<T/2 (in the above formula, T is the total thickness of the film),
The present invention relates to a biaxially stretched single-layer film of polyethylene terephthalate or a polyester having ethylene terephthalate units as a main component, characterized in that the crystallinity (xc) of the film is xc≧35%. Both the abrasion resistance and thermal dimensional stability of the film are closely related to the film's surface hardness.Moreover, the lower the surface hardness, the better the abrasion resistance, while the higher the surface hardness, the better the thermal dimensional stability. It was found that dimensional stability was improved. White powder is generated in the magnetic tape manufacturing process, especially during the polishing process using a calendar roll after applying the magnetic film, and the surfaces that require wear resistance are limited to those to which the magnetic film is not applied. Therefore, by imparting abrasion resistance to one side of the film and supplementing dimensional stability to the other side, a satisfactory magnetic tape can be obtained. The present invention is based on this basic knowledge, and consists of a polyester film in which one surface of the film has high hardness and the other surface has low hardness. and the higher surface hardness (Hh)
(unit: Kg/cm ² , the same applies hereinafter) is 530Hh<880, and the hardness (Hl) of the lower surface is Hl<500. The thickness of the latter layer with low hardness is less than 1/2 of the total thickness of the film, and the crystallinity of the film is 35% or more. The surface hardness of a film is expressed by the resistance it exhibits when a standard substance is injected into it.In the present invention, the film is scratched with a spherical indenter, and the resulting grooves are formed into spherical indentations whose diameter is the width of the groove. The surface hardness is calculated by dividing the applied load when scratching by the surface area of the above spherical depression, assuming that it is continuous, and at a temperature of 20℃.
It was measured as follows. FIG. 1 is a schematic perspective view of an apparatus for measuring surface hardness, in which a film 1 to be measured is stretched over a metal window-frame-shaped frame 2, and this frame 2 is placed on a rail 3 and driven As the motor 4 rotates, the string 5
is moved in the direction of arrow A at a constant speed (10 mm/min). On the other hand, a spherical indenter (manufactured by Saphire) 7 attached to the tip of the support 6 is pressed against the surface of the film, and the load is kept constant by a weight 8. Note that 9 is an extender for moving the spherical indenter up and down, and 10 is a guide tool that guides the support 6 so that it can move up and down. When the film is moved at a constant speed while being pressed by an indenter with a constant load, a scratch groove with a width d and a cross section as shown in Fig. 2 is formed. Considering that a spherical depression with a diameter of d has been created,
Surface hardness (H) can be calculated from the following formula according to the Brinell hardness calculation formula. In the above formula, W is the load (Kg), γ is the radius of the spherical indenter (cm), and d is the width of the scratch groove (cm).
Since the hardness H calculated by equation (1) will be different if W and γ are different, the hardness of the front and back sides of the film in the present invention is as follows: W is 5×10 ^-3 Kg, γ is This is the value when measured as 5×10 ^-3 cm. The crystallinity (xc) of the film is carbon tetrachloride-n
- Calculated from the density of the film measured by the density gradient tube method using a heptane mixture using the following formula. xc = ρ - ρa / ρc - ρa × 100 In the above formula, ρ is the density of the sample, ρc is the crystal density (1.455 in the case of polyethylene terephthalate), ρ
a is the non-product density. The thickness (tl) of the layer with low surface hardness of the film is measured as follows. That is, using the apparatus shown in Fig. 1, we varied the ratio W/γ of the load (W) to the radius of the indenter (γ) in the range of 1.0 to 3.5, and selected conditions that allowed good measurement. The depth of the groove when the hardness calculated using equation (1) is 500 (W/γ) ^-0.06 , ^that is,

is the value of [formula]. Here, 500 is the upper limit value of Hl, and (W/γ ^{) -0.06} is a correction term due to changing W and γ. According to experiments, even if the film is the same, when measured under conditions where the value of W/γ is large, the hardness appears to be smaller than when measured under conditions where the value of W/γ is small.
It is necessary to include this correction term. At this time, W/γ is
If W/γ is smaller than 1.0, sufficient scratches will not be obtained for measurement, and if W/γ is larger than 3.5, hunting of the indenter or film breakage may occur, making measurement impossible, so pay attention to the measurement conditions. There is a need. In the film of the present invention, it is not enough that there is a difference in hardness between the front and back surfaces; the thickness (tl) of the layer with lower hardness is closely related and becomes an important factor. In other words, tl must be smaller than 1/2 of the film thickness T, and if it is more than 1/2, it will not provide sufficient dimensional stability and will not only be unsatisfactory as a magnetic tape, but also due to the difference between the front and back sides. Curling tends to occur, which is undesirable. The hardness (Hl) of the surface with lower hardness must be 500 or less; if it exceeds 500, the wear resistance of this surface will deteriorate extremely. On the other hand, the hardness of the higher hardness (Hh)
must be 530 or more and less than 880; if it is less than 530, sufficient dimensional stability cannot be imparted, and 880
If the temperature exceeds that level, the surface becomes too hard to be used. Preferably, Hl is 400 or less, and Hh is preferably 580 or more. When the crystallinity of the film is less than 35%, the dimensional stability is poor, causing problems in the magnetic film coating process. The polyester film of the present invention has polyethylene terephthalate or ethylene terephthalate units as its main component and copolymerized with other acid component monomers and glycol component monomers to the extent that their properties are not impaired, or these polymers or copolymers are used. A blend containing 70% by weight or more of ethylene terephthalate is used. These resins may further contain inorganic and organic particles in an amount that does not impair the properties of the film and does not separate from the film, if necessary. Next, a method for manufacturing the film of the present invention will be explained. The surface hardness of the film is determined by the length of the film,
It varies depending on the temperature during transverse stretching, the stretching ratio, and the temperature during heat treatment, and lowering the stretching temperature, increasing the stretching ratio, and lowering the heat treatment temperature leads to lower hardness. Hardness is usually adjusted by a combination of these. In order to provide a hardness difference between the front and back surfaces of the film of the present invention, the temperature during stretching and/or the temperature during heat treatment may be made different for each of the front and back surfaces. Since the film hardness is also related to the amount of other copolymers (for example, the amount of diethylene glycol used), the intrinsic viscosity of the film, etc., the film of the present invention can be obtained by combining these and the above-mentioned heating temperature conditions. Preferably, the heat treatment temperature is 210° C. or higher in terms of dimensional stability, and any deficiency in wear resistance can be corrected by providing a difference in stretching temperature between the front and back surfaces. The means to create a temperature difference between the front and back sides include preheating in the stretching area, heating to different temperatures on the front and back sides using rolls, infrared heaters, or other heating methods during the stretching process, and blowing hot air onto the film in the heat setting area. By methods such as changing the amount and/or temperature. When the stretching process is carried out in several stages, the temperature difference may be applied within several stages, in part (for example, only in one stage), or in all. When heat treatment is also performed in several stages, a temperature difference is similarly applied over some or all of the stages. Further, the same method can be used when performing a relaxation heat treatment to a degree that does not impair the quality of the film. The film of the present invention is biaxially stretched, and is first stretched in the longitudinal direction with a roll stretching machine and then in the width direction with a stenter, or first stretched in the width direction with a stenter and then stretched in the longitudinal direction with a roll. It can be applied to any method, such as a method of stretching in the longitudinal direction and the width direction simultaneously using a stenter. Furthermore, as a method of imparting a hardness difference between the front and back sides of the film, it is also possible to melt-extrude the film into an amorphous sheet, apply cooling temperature at that time to a difference between the front and back sides, and then biaxially stretch the film. However, since this method tends to cause thick spots, it cannot be said to be a preferable method in this respect, but it can be adopted if optimal conditions can be selected. Next, an example of the method for manufacturing the film of the present invention will be described. The abrasion resistance of the film obtained in this example is measured as follows. For this measurement, a measuring device consisting of the mechanism shown in FIG. 3 is used. In the figure, 11 is a test film, 12 is a free roll, and 13 is a non-rotating wear roll which is chromium plated (less than 0.5S) and is reciprocated up and down as shown by the arrow. 14 is a weight of 100 g, which pulls the film 11 downward. Sample film 11 is 10mm wide
The abrasion roll 13 is moved up and down for 40 hours in an atmosphere of (30±2)° C. and (40±5)% RH to cause abrasion with the film. The wear powder generated at this time was washed away by ultrasonic cleaning in ethanol, the ethanol was evaporated, and 1N KOH
Add 2 c.c. of aqueous solution and hydrolyze under conditions of 10 atm and 180 to 190°C for 7 to 8 hours. After hydrolysis, it is placed in a microcell and the absorbance of terephthalic acid is determined. The degree of abrasion resistance is determined based on this absorbance. Example 1 A polyethylene terephthalate chip with an intrinsic viscosity of 0.625 was extruded into a sheet from a die set at 290°C using an extruder, and the sheet was made into an amorphous sheet by contacting it with a casting roll adjusted to 70°C, and then a film piece was rolled. The other side is heated to 75℃ using an infrared heater, and the other side is heated to 85℃ using an infrared heater.
After stretching to 4.0 times, it is rapidly cooled. Next, this uniaxially stretched film is stretched in the width direction using a known stenter.
It was stretched 3.4 times at 90°C and sent to a heat setting area, where it was heat set at 230°C on the side that had been stretched at 85°C and at 213°C on the side that had been stretched at 75°C. After this heat treatment, the film was slightly loosened in the width direction, and a film finally having a thickness of 22 μm was obtained. The thickness of the low hardness layer of the film obtained was less than 1/2 of the total thickness of the film. The properties of the obtained film are shown in Table 1 below. Comparative Example 1 The same procedure as in Example 1 was carried out except that both sides of the film were heated to 85°C and stretched 4.0 times in the longitudinal direction, and both sides of the film were heat-treated at 230°C in the heat treatment area. The properties of the obtained film are shown in Table 1. Although the thermal dimensional stability was excellent, the abrasion resistance was poor. Comparative Example 2 The same procedure as in Example 1 was performed except that in the heat treatment region of Example 1, the film surface heated and stretched at 75°C was heat-treated at 180°C, and the film surface heated and stretched at 85°C was heat-treated at 230°C. The thickness of the low hardness layer on the lower hardness side of the obtained film accounted for more than half of the total thickness of the film. As shown in Table 1, this film had good abrasion resistance, but had a large heat shrinkage rate, making it unsuitable for use in magnetic tapes, and the film did not have good flatness, such as curling. Example 2 A polyethylene terephthalate chip with an intrinsic viscosity of 0.650 was produced using an ordinary extruder at a die temperature of 295
℃ and then melt-extruded into a sheet shape, brought into contact with a casting roll controlled at 70℃, and quenched.The obtained amorphous sheet was heated to 80℃, and was rolled in the longitudinal direction by 4.5℃ using the difference in peripheral speed of the rolls. Stretched twice. This uniaxially stretched film is heated at 105℃ in the width direction using a stunter.
After stretching to 3.5 times, heat treatment is performed by setting the temperature of one heat treatment area to 238℃ on one side and 213℃ on the other side, and in the other heat treatment area, both sides are heated.
Set the temperature below 180℃ to pass through, then after cooling,
The film was wound up to obtain a film with a thickness of 7.5 μm. The properties of this film are shown in Table 2 below. One side of this film was quite hard, and there were concerns about wear resistance, but there were no problems in the normal magnetic tape manufacturing process, and the film did not break even though the heat treatment temperature was quite high. In addition to a good yield in the film forming process, it also exhibited excellent dimensional stability.
Further, the thickness of the layer with lower hardness of this film was less than half the thickness of the entire film. Comparative Example 3 The stretching temperature and stretching ratio in the longitudinal direction were 85°C and 85°C, respectively.
A film was formed in the same manner as in Example 2 except that the film was multiplied by 4.0 times. The properties of the obtained film are shown in Table 2.
It was found that this film had poor abrasion resistance during the magnetic film coating process and lacked performance as a magnetic tape. The thickness of the low hardness layer of this film was less than half of the total thickness of the film, but the lower hardness value was 510. Example 3 In the process shown in Example 2, the heat treatment area was set at 238°C at 218°C, the line speed was increased to about 1.6 times that of Example 2, and the heat treatment time was reduced to about 3 seconds. A film was formed by reducing the shrinkage and increasing the stretching speed in the width direction, but otherwise operating in the same manner as in Example 2. The properties of the obtained film are shown in Table 2. Although this film had a low degree of crystallinity, its shrinkage rate was not high enough to cause problems in the production of ordinary magnetic tapes, and the thickness of the low hardness layer was less than half of the total thickness of the film. Comparative Example 4 The steps shown in Example 4 were as in Example 2 except that the stretching ratio in the width direction was 3.0 times, and the portions of the heat treatment area that were set at 238°C and 213°C were set at 200°C and 180°C, respectively. A film was formed in the same manner. The properties of the obtained film are shown in Table 2. Although the crystallization of this film was the same as in Example 3, the shrinkage rate in the longitudinal direction was 7.5% and the shrinkage rate in the width direction was 5.2%, and the yield in the magnetic film coating process was _extremely low.
The value was also not satisfactory.

【table】

[Table] In the above table, side A and side B indicate the low-temperature side and the high-temperature side of the film in which a temperature difference was created between the front and back sides of the film during stretching and heat treatment. Thermal shrinkage rate is due to hot air heating, MD is the value measured along the extrusion direction (longitudinal direction) of the film, and TD is the value measured along the width direction of the film.

[Table] The above explanations and examples shown in the examples are representative examples to aid understanding of the present invention, and the film of the present invention is not limited by these and does not fall within the scope of the invention. Other changes and modifications may be made.

[Brief explanation of the drawing]

Fig. 1 is a schematic perspective view of the device used to measure the surface hardness of the film of the present invention, Fig. 2 is a cross-sectional view of the spherical indenter and the film during surface hardness measurement, and Fig. 3 is a measurement of the abrasion resistance of the film. FIG. 2 is an explanatory diagram showing the mechanism of the device used for this purpose. In the figure, 1 is the film sample to be measured, 2 is the metal frame,
3 is a rail, 4 is a drive motor, 7 is a spherical indenter,
8 is a weight, 11 is a sample film, 12 is a free roll, 13 is a wear roll, and 14 is a weight.

Claims (1)

[Claims] 1. Surface hardness (Hh) of one side of the film (Kg/
cm ² ) is 530Hh<880, the surface hardness (Hl) (Kg/cm ² ) of the other side is Hl500, and the thickness (tl) of the layer with low surface hardness is tl<T/2 (in the above equation, T is the total thickness of the film),
A biaxially stretched single-layer film of polyethylene terephthalate or a polyester containing ethylene terephthalate units as a main component, characterized in that the crystallinity (xc) of the film is xc≧35%.